1. Field of the Invention
The present invention generally relates to sheetmaking control systems and, more particularly, to sheetmaking control systems wherein measuring devices scan across travelling sheets.
2. State of the Art
It is well known that on-line measurements can be made to detect properties of sheet materials during manufacture. Generally speaking, on-line measurements are made to enable prompt control of sheetmaking processes and, thus, to assure sheet quality while reducing the quantity of substandard sheet material which is produced before process upset conditions are corrected. In the papermaking art, for instance, on-line sensors can detect variables such as basis weight, moisture content, and caliper of paper sheets during manufacture.
One of the main complications in making on-line measurements during sheetmaking is that the physical properties of sheet materials usually vary in the machine direction as well as in the cross direction. (In the sheetmaking art, the term "machine direction" refers to the direction of travel of sheet material during manufacture, and the term "cross direction" refers to the direction across the surface of a sheet perpendicular to the machine direction.)
To detect variations in sheet materials, it is well known to use scanning sensors that periodically traverse back and forth across a sheetmaking machine in the cross direction while detecting values of a selected sheet property along each scan. Normally, the sheet being produced is traversed from edge to edge during each scan. The time required for a typical scan is generally between about twenty and thirty seconds for conventional scanners. The rate at which measurement readings are provided by such scanners is usually adjustable; a typical rate is about one measurement reading every fifty milliseconds.
In practice, measurement information provided by scanning sensors is usually assembled after each scan to provide a "profile" of the detected sheet property in the cross direction. In other words, each profile is comprised of a succession of sheet measurements at adjacent locations in the cross direction. The purpose of the profiles is to allow cross-directional variations in sheet properties to be detected easily. Based upon the detected cross-directional variations in the detected sheet property, appropriate control adjustments may be made to the sheetmaking machine with the goal of reducing profiles variations both in the cross direction and in the machine direction.
Although modern sheetmaking control systems provide substantial advantages, there are some shortcomings. One shortcoming of conventional systems is that their response times are relatively slow, especially following abrupt change in process conditions such as caused by sheet breaks or real changes, or during start-up. The slow response times of the control systems, although necessary to assure control stability, often allow substantial quantities of substandard sheet material to be produced before effective corrective actions are implemented. Thus, it can be appreciated that there is a need for control systems that rapidly adjust sheetmaking machines when process conditions change abruptly but, under normal conditions, provide smooth operation.